Better Never Than Late

By Paul Hetzler, Horticulture and Natural Resource Educator

It’s not too early to start
thinking about late blight. No relation to early blight, with which it shares a
last name, late blight has become a perennial disease since infected tomato
plants were shipped from southern greenhouses to the Northeast in May 2009.
Prior to that, late blight was uncommon, but now we seem to be able to bank on
its arrival each August. The fact that it is a seasonal immigrant is worth
noting, since most garden diseases (such as early blight) are already here in
the soil.

Gardeners and produce growers
make a fuss about late blight because it has the potential to kill acres of
tomatoes and potatoes in a matter of days; its fearsome reputation is
well-deserved. Given the botanical name Phytophthora infestans, “highly
contagious plant destroyer,” it is what laid waste to the Irish potato crop
from 1844 to 1846, leading to a devastating famine.

To be fair to a right nasty
microbe, however, late blight was not entirely to blame for the Great Hunger of
1845-1852. During this period, record quantities of Irish beef, pork, mutton,
and grain were shipped to England. The fact that potatoes were the only
foodstuff not somehow limited or confiscated by the occupying British forces is
the reason the failure of that single crop caused the death of over a million
Irish, and the emigration of even more.

Even though we have the luxury
of growing other vegetables, we still want to avoid late blight. How to dodge a
killer that breezes into town on the wind is a fair question, though. It turns
out that the late blight fungus is technically a water mold, and requires
moisture for a spore to germinate on a tomato or potato leaf. Planting tomatoes
far apart—24 inches is good—and making wide row spacing—say, 36 inches—will improve
air circulation and sunlight penetration. Most gardeners already stake or
trellis tomatoes; this is also key. Pruning is less common, but worthwhile.
Maintain a single stem by pinching off “suckers” that arise at every leaf
junction (axil). This will help reduce disease, plus get you larger tomatoes
that start to ripen earlier.

Home gardeners can use
protective sprays to stave off late blight, but none of these products will
stop the disease once it hits. Fungicides with the active ingredient chlorothalonil
are readily available, and organic growers can use copper-based fungicides. To
avoid needless spraying, usablight.org has a great tool for both home and
commercial growers. You can sign up to receive alerts when late blight is found
elsewhere in the region, and can set the distance you prefer. Weather plays a
big part in how it spreads. Not only are wet conditions conducive for the
pathogen to get started on a plant, cloud cover shields late blight spores from
UV radiation. In full sun, spores are killed in about an hour, but if it is
overcast they are good for many days.

Knowing how to spot the scourge
is critical. Many diseases of tomatoes and potatoes can be mistaken for late
blight. Early blight, a ubiquitous garden disease, is soil-borne and begins on
the bottom leaves, working its way up the plant. Septoria leaf spot, another
soil-borne pathogen, sometimes occurs along with early blight. Blossom-end rot,
which actually is due to severe water stress and is not a disease, causes
tomatoes to become blackened and rotten on the bottom.

The first obvious symptoms of
late blight are large, watery lesions on leaves, giving them the appearance of
having been frozen and then thawed. In moist and/or humid conditions, white
fungal growth may be seen at the margins of the lesions. Since late blight is
airborne, symptoms will show up throughout the plant, not just near the bottom.
The disease also affects stems, sometimes killing the plant above the point of
infection. On the tomatoes themselves, late blight causes large, brown,
greasy-looking patches that are surprisingly firm to the touch.

If late blight is confirmed, you
can try and salvage unripe tomatoes of mature size by immersing them in a 10%
bleach solution and laying them out on a counter top or baking sheet where you
can keep an eye on them. Discard any fruit that develop lesions. Ripe tomatoes
with small late blight spots are safe to eat after removing affected areas, but
the USDA recommends not using them for canning.

Obviously, late blight can spread
on the wind from one garden or farm to the next. To protect other growers,
diseased plants should be placed in clear plastic bags and left in the sun
until no green tissue is left. Once the plants are completely dead, the late
blight organism can no longer produce spores and it is safe to compost those
plants, bury them, or discard them in the trash.

In northern latitudes, the only
way late blight has been able to overwinter is by hiding in potatoes from
infected plants that have been left in the ground. When these volunteers sprout
in the spring, late blight can work its way up the stem, produce spores and
touch off an epidemic. Now that we are in an age of recurring late blight
infestations, it is essential for gardeners and farmers scout for and destroy
all volunteer potato plants each year.

For more information on late
blight, go to usablight.org, or call your local Cornell Cooperative Extension
office. Let’s do all we can this year to keep it late. Or better yet, never.